Putative Anticancer Compounds from Plant-Derived Endophytic Fungi: A Review

Endophytic fungi are microorganisms that exist almost ubiquitously inside the various tissues of living plants where they act as an important reservoir of diverse bioactive compounds. Recently, endophytic fungi have drawn tremendous attention from researchers; their isolation, culture, purification, and characterization have revealed the presence of around 200 important and diverse compounds including anticancer agents, antibiotics, antifungals, antivirals, immunosuppressants, and antimycotics. Many of these anticancer compounds, such as paclitaxel, camptothecin, vinblastine, vincristine, podophyllotoxin, and their derivatives, are currently being used clinically for the treatment of various cancers (e.g., ovarian, breast, prostate, lung cancers, and leukemias). By increasing the yield of specific compounds with genetic engineering and other biotechnologies, endophytic fungi could be a promising, prolific source of anticancer drugs. In the future, compounds derived from endophytic fungi could increase treatment availability and cost effectiveness. This comprehensive review includes the putative anticancer compounds from plant-derived endophytic fungi discovered from 1990 to 2020 with their source endophytic fungi and host plants as well as their antitumor activity against various cell lines.


Introduction
In 1866, de Bary introduced the term "endophyte" [1]. An endophyte may be a fungal or bacterial microorganism that colonizes various interior parts of plants causing no apparent pathogenic effects on its host plants. The endophytes, most commonly endophytic fungi, are believed to help plants adapt to abiotic factors (high temperature and salinity, drought, metal toxicity, and harmful effects of light) as well as biotic factors (herbivores, insects, nematodes, and pathogens). This is mainly achieved by the secondary bioactive metabolites produced by the endophytic fungi. In their symbiotic relation, the endophytes are fed and protected by the host plant, and in return, these microorganisms produce bioactive secondary metabolites, enhancing the growth of the host plant and protecting the plant from pathogens and herbivores [2]. Therefore, endophytic fungal metabolites can also be exploited as drugs for the treatment of various types of human diseases, including cancer [3].

Anticancer Activity of Endophytic Fungi
Endophytic fungi have been a known source of anticancer agents since the discovery of the valuable drug Taxol (also known as paclitaxel, a diterpenoid) isolated for the first time from an endophytic fungus Taxomyces andreanae obtained from the Pacific Yew bark (Taxus brevifolia) [6]. Since then, other anticancer drugs have been isolated from endophytic fungi, and among these 9-methoxycamptothecin and 10-hydroxycamptothecin from Fusarium solani [10], camptothecin from Entrophospora infrequens [11]; the anticancer lead compounds podophyllotoxin from Phialocephala fortinii [12] and deoxypodophyllotoxin from Aspergillus fumigatus [13] fueled further research on endophytic fungi to discover many other important known and novel anticancer compounds. According to this review, until now, more than 100 different fungal species have been identified to produce more than two hundred putative anticancer compounds (Figures 1 and 2) reported to possess antiproliferative and/or cytotoxic properties against more than 60 different cell lines (Tables 1-3). Figure 1 indicates that endophytic fungal-derived anticancer agents gained attention from scientists over the past three decades. Meanwhile, Figure 2 represents the abundance of different chemical classes and diversity of fungal metabolites. The anticancer compounds isolated from endophytic fungi are effective against diverse cell lines that could be helpful in combating any particular type of cancer (Table 1).

Hep3B
Human hepatoma cell line PC-3 M Metastatic prostate cancer HM02 Human gastric carcinoma RAW264. 7 Mouse macrophage cell

HL251
Human lung cancer SW480 Human colon cancer cells

HL-7702
Normal hepatocyte SW-620 Colon tumor cell line HLK 210 Human leukemia SW1116 Human colon cancer cell line HCT- 8 Human colorectal adenocarcinoma SW1990 Human pancreatic cancer cells HCT- 116 Colon tumor cell line T24 Bladder carcinoma H22 Hepatic cancer cells in mice T47D Breast cancer

H1975
Non-small-cell lung cancer cells/human lung adenocarcinoma THP-1 Human monocytic cell line

Anti-Cancer Agents in Clinical Use Shared by Plants and Endophytic Fungi
Plants are prolific sources of anticancer agents. In the area of cancer, of the 175 approved small molecules over the years from the 1940s to 2014, 75% (131) are other than synthetic and 49% (85) are either natural products or their derivatives [222]. Very recently, it was reported that among the approved 321 anticancer molecules from all sources during the period of 1946 to 2019, 35 (10.9%) were unaltered natural products and 65 (20.2%) were natural product derivatives compared to 53 (16.5%) completely synthetic drug molecules. Some of these agents obtained from plants are also found in their corresponding endophytic fungi. The following are some examples of plant/endophytic fungi-derived cancer effective agents [1,6] (Figure 3a,b).
Paclitaxel (Taxol ® ) is used in combination with other anti-cancer drugs in ovarian, breast, non-small cell lung cancer (NSCLC), and Kaposi sarcoma. An active paclitaxel analogue, docetaxel is used in breast and non-small cell lung cancer (NSCLC) treatment [223]. Even though camptothecin exerted severe bladder toxicity in its clinical trial in the 1970s and therefore, was dropped, its two water-soluble derivatives, topotecan and irinotecan, have been shown to be more effective anti-cancer agents and are being utilized for these purposes [223]. Topotecan (Hycamtin ® ) was the first CPT derivative that was orally available and has been approved for cervical (when used in combination with cisplatin), ovarian, and non-small cell lung cancer treatment. Irinotecan (Camptosar ® ) has been approved for colorectal cancer treatment. These agents show cytotoxicity on account of their ability to inhibit a fundamental enzyme, topoisomerase-I, involved in the winding and unwinding process of DNA during replication or protein synthesis [1,223]. The vinca alkaloids, vinblastine and vincristine, and their semi-synthetic analogs, vinorelbine and vindesine, are primarily used in combination with other chemotherapeutic drugs in the treatment of advanced testicular cancer, breast cancer, Kaposi's sarcoma, lung cancer, leukemias, and lymphomas [223]. Etoposide and teniposide are clinically effective semi-synthetic derivatives of a podophyllotoxin isomer, epipodophyllotoxin, which are used in bronchial cancers, lymphomas, and testicular cancer treatments [223].

Depsidones
Botryorhodines A (12a) and B (12b), two depsidones, were isolated from the endophytic fungus Botryosphaeria r. associated with Bidens pilosa. These compounds exhibited weak antitumor activity against the HeLa cell line at a concentration of 96.97 and 36.41 µM, respectivel [48]. Depsidone 1 was discovered from a fungus of the Pleosporales order (BCC 8616) isolated from an unidentified plant leaf of the Hala-Bala forest origin. Depsidone 1 displayed weak cytotoxicity to KB and BC cell lines with IC50 values 6.5 and 4.1µg/mL, respectively [43] (Table 2).

Ergochromes
Phomopsis l., an endophytic fungus of Dicerandra frutescens, produced three compounds dicerandrols A, B, and C (15a-15c), structurally related to the ergochromes and secalonic acids as they also have the same tricyclic C15 system with a similar arrangement of substituents. These compounds displayed modest antitumor activities toward A549 (lung adenocarcinoma epithelial cell line) and HCT-116 (colon tumor cell line) cell lines [132] ( Table 2).
Secalonic acid D (16), isolated from mangrove plant endophytic fungus no. ZSU44, displayed potent cytotoxicity against HL60 (the human promyelocytic leukemia cell line) and K562 (human leukemia cells) cells with IC50 values of 0.38 and 0.43 µM, respectively. It caused apoptosis in those cell lines and cell cycle arrest in the G(1) phase as well [158].
Radicicol (19) was obtained from Chaetomium c. associated with Ephedra fasciculate and it is a HSP90 (heat shock protein) inhibitor, which is frequently expressed highly in cancer cells. It also showed cytotoxicity to the MCF-7 (breast cancer) cell line at an IC50 value 0.03 µM [55].

Peptides
Leucinostatin A was isolated from the endophyte Acremonium spp. associated with Taxus baccata and was shown to be effective against BT-20 (breast cancer) cell line with an LD50 value of 2 nM [14]. It inhibits the growth of prostate cancer cells through the suppression of IGF-I (Insulin-Like Growth Factor-I) expression in PrSC (prostate stromal cells) [228] (Table 2).
Sequoiatone A (32a) and B (32b), two novel polyketides (Figure 3b), were isolated from a Sequoia sempervirens bark endophyte, Aspergillus p. These polyketide compounds were tested against 60 diverse human tumor cell lines, and among them, breast cancer cell lines showed the greatest sensitivity [37] (Table 2).

Recently Reported Metabolites with Potential Cytotoxicity and the Case of Fusarubin
More than one hundred metabolites have been isolated and evaluated for putative anticancer activities in the years 2018 to 2020. Cytotoxic activities of these endophytic metabolites have been summarized in Table 3. Among the reported metabolites, penicolinate A isolated form Bionectria spp. [159] and pyrrocidine A isolated from Cylindrocarpon spp. [166] exhibited potent cytotoxicity against against the human ovarian cancer cell line A2780. Fusarithioamide B, a new type benzamide, isolated form Fusarium c., showed potent activity against several cell lines [160]. 3-(4-nitrophenyl)-5-phenyl isoxazole was reported to have a potent effect against HepG2 and SMCC-7721 cells [161], while spiciferone F was reported to have a strong effect against MCF7 [162]. Liu et al. isolated two metabolies, namely xylariphthalide A and cis-4-hydroxy-6-deoxytalone, and Sharma V. et al. isolated Xylarolide A from Diaporthe spp. [163,164]. All these metabolites showed activity towards cancer cells. Three naphthaquinones, anhydrofusarubin, fusarubin, and 3-deoxyfusarubin, and one aza-anthraquinone, bostrycoidin, have potentiality as bioactive compounds against cytotoxicity on vero cells. These metabolites were isolated from a Fusarium s. strain isolated from Casia alata. [8]. Monolinolein, bafilomycin D, and 3 -hydroxydaidzein displayed a strong effect against A549 cells. These metabolites were isolated from actinomycete strain YBQ59 residing in Cinnamomum cassia [167]. Colletotrichum g. A12 produced colletotricone A, which showed moderate activity against MCF-7, NCI-H460, HepG-2m and SF-268 tumor cell lines [168]. Mollicellin G, a depsidone, was reported as a moderately active cytotoxic metabolite towards HepG2 and Hela cells [169]. A metabolite of Pestalotiopsis spp., named demethylincisterol A3, showed potential cytotoxicity against human cancer cell lines Hela, A549, and HepG [170].
Fusarubin and anhydrofusarubin have been isolated from the endophytic fungi Cladosporium residing inside Rauwolfia leaves. These compounds inhibited the cell growth of different leukemia cell lines (OCI-AML3, HL-60, U937, and Jurkat) by arresting the cell cycle and augmenting apoptosis. Whereas fusarubin exerted an antiproliferative effect on OCI-AML3 cells by up-regulating p21 in a p53-dependent manner, apoptosis was induced only in a small sub-population of leukemic cells by inducing the production of the Fas ligand ( Figure 4) [9].  Table 3. Fusarubin and anhydrofusarubin have been isolated from the endophytic fungi Cladosporium residing inside Rauwolfia leaves. These compounds inhibited the cell growth of different leukemia cell lines (OCI-AML3, HL-60, U937, and Jurkat) by arresting the cell cycle and augmenting apoptosis. Whereas fusarubin exerted an antiproliferative effect on OCI-AML3 cells by up-regulating p21 in a p53-dependent manner, apoptosis was induced only in a small sub-population of leukemic cells by inducing the production of the Fas ligand ( Figure 4) [9].

Conclusions
Several hundred endophytic fugal metabolites have been isolated to have cytotoxic and antimicrobial effects. Many metabolites are currently available as drugs on the market. Given that plants host endophytes as part of a symbiotic relationship, some plant metabolites might have an endophytic fungal origin. In fact, increasing evidence indicates that some of these plant metabolites are also produced by fungi. Many of the isolated metabolites of endophytic fungi inhabitant medicinal plants have been proved to have cytotoxic effects in vitro. Several of these compounds have been investigated at the molecular level to elucidate the mechanism, since these metabolites are produced in very small quantities by endophytes of plant origin. Due to very insignificant yields and isola-

Conclusions
Several hundred endophytic fugal metabolites have been isolated to have cytotoxic and antimicrobial effects. Many metabolites are currently available as drugs on the market. Given that plants host endophytes as part of a symbiotic relationship, some plant metabolites might have an endophytic fungal origin. In fact, increasing evidence indicates that some of these plant metabolites are also produced by fungi. Many of the isolated metabolites of endophytic fungi inhabitant medicinal plants have been proved to have cytotoxic effects in vitro. Several of these compounds have been investigated at the molecular level to elucidate the mechanism, since these metabolites are produced in very small quantities by endophytes of plant origin. Due to very insignificant yields and isolation difficulties, these secondary metabolites may not be available to carry out in vivo studies in animal models. Some laboratories applied synthetic approaches to produce natural product derivatives, and one group also tried to synthesize some of these compounds. Optimizing derivatization and synthetic approaches is critical to attain higher yields for animal studies. These approaches will be key for investigating and developing these putative anticancer compounds into treatments.